COURTSHIP AND SEX 241 



ditions were such that they conduced to early breeding, 

 without there being specially rich nutrition. The repro- 

 ductive functions were " forced " without being richly fed. 

 These birds produced males in great excess. In the other 

 aviary the birds were rather kept back ; they nested later 

 and moulted later ; their reproductive functions were not 

 stimulated ; the ova matured more slowly and were at the 

 same time more highly fed. These birds produced a marked 

 excess of females. It is probable that the environmental 

 differences operated by selective action on the germ-cells 

 produced. 



(V.) Mendelian View of Sex. — If sex, like many a morpho- 

 logical character, is due to some factor or determiner in a 

 chromosome, which is called the accessory, sex, or X-chromo- 

 some, then there are three possible Mendelian interpretations, 

 (i) Both males and females may be heterozygous — male 

 D(R) X female D(R). (2) The male may be heterozygous, 

 D(R), and the female a homozygote recessive — male D(R) X 

 female R. (3) The female may be heterozygous, and the 

 male a homozygote recessive — male R X female D(R). This 

 third interpretation is strongly supported by certain cases 

 that have been carefully studied, especially the currant moth 

 and the canary. 



To suppose that the female canary is heterozygous, 

 means that it will have equal contingents of male-producing 

 and female-producing ova. The male bird by hypothesis 

 will have homogeneous spermatozoa, none bearing the 

 femaleness factor. The two contingents may be represented 

 on this scheme : — 



The chance combinations are 2FM -{- 2MM ; and this 

 means equal numbers of females and males, if femaleness 

 is dominant to maleness. 



From his observations on the germ-cells of fowls 



R 



